This invention relates to capacitance-type fluid measuring apparatus or gages and more particularly to means for compensating for errors in such gages so as to provide for an indication of the mass of a fluid within a container or containers within a close degree of accuracy.
In the conventional rebalancing bridge-type of capacitance gage, a measuring capacitor having shaped electrodes is immersed in the fluid in a container. The change in capacitance due to a change in fluid level is used to provide an indication of the level of the fluid in the container or the mass of the fluid remaining in the container. This is accomplished by connecting the measuring capacitor across one arm of a bridge circuit and a reference capacitor across an opposing arm of the bridge circuit in which a pair of voltage sources comprise the other two arms of the bridge circuit. A phase sensitive detector responsive to the bridge output is coupled to a rebalancing motor to vary the voltage applied to the reference capacitor thereby maintaining the bridge balance.
When a gage of the above type is employed to measure the quantity of fluid in a tank such as the quantity of fuel in an aircraft fuel tank, the gage readout or indicator is calibrated in terms of mass or weight of the fuel remaining in the tank. It is standard procedure to shape or profile the electrodes of the measuring capacitor such that the dry capacitance of the immersed portion is at all times propertional to the corresponding volume of fluid in the tank. In that manner the gage will provide an accurate indication of weight of fuel so long as the capacity index (k-1)/D, where k and D are the relative dielectric constant and density of the fuel respectively, is a constant. Unfortunately, the capacity index is not a constant considering the types of fuels used and the temperature ranges to which they are subjected, so that some means is required to compensate for capacity index variation.
One means for providing compensation for the above-mentioned variation in capacity index is an auxiliary sensing capacitor immersed in the fluid to be gaged, usually close to the bottom of the fluid container, and a compensating circuit which cooperates with the auxiliary sensing capacitor. Compensating means of this type are described in the patent to Stanley J. Smith, U.S. Pat. No. 3,022,665 and in a more recent patent to Rudolph Ehert et al. U.S. Pat. No. 4,021,707. Because of temperature stratification of fuel in aircraft fuel tanks, whereby relatively cool fuel sinks to the bottom so that the auxiliary sensing capacitor does not see a truly representative sample of the fuel within the container, these compensation means are not fully satisfactory. Another method for providing compensation for capacity index variation is described in the patent to Alfred D. Gronner, U.S. Pat. No. 3,295,372. According to that method, compensation starts by approximating the relationship (k-1)=(AD).sup.n, where k and D are as described above, A is a proportionality constant and n is an exponent. According to the patent a capacitor of a predeterminable correct value is placed in series with the measuring capacitor so that this series combination can be profiled to a certain hyperbolic function of the capacity of the measuring condenser chosen. A constant amplitude alternating voltage is applied to the combination. Assuming a constant fuel dielectric constant the current flowing in the capacitor combination will be a linear function of the height of the fluid and the fluid container. If a linear readout is desired, a rebalance potentiometer is characterized with the same function.